The Vector Nature of Newton's Second Law

A block of mass 2.0kg is placed on a smooth plane, inclined to the horizontal at an angle of 15degrees. The force of gravity, acting straight down on the block, is 20N.
a) What is the acceleration of the block down the plane?

I'm having trouble with this because I know:
Net F=fgcostheta-friction
or fgcostheta-friction=ma=2a